1. Field of the Invention
The present invention is directed to the field of intensity control for light emitting display devices and more specifically to the area of providing electronic dimming control of the light intensity produced by vacuum fluorescent displaY (VFD) devices.
2. Background Information
The use of VFD devices in automotive vehicles has been widely adopted as a reliable and efficient means for providing display information in a crisp format suitable for reading in both day and night conditions.
In attempting to control the brightness of such displays, it has long been appreciated that pulse width modulation techniques can be used with satisfactorY results. Pulse width modulation provides for various electrodes of the VFD to be selectively energized at a frequency level higher than that which is visually perceivable but with a duty cycle that is varied to achieve a desired brightness level.
U.S. Pat. No. 4,358,713 illustrates a brightness control circuit for a VFD in which a constant frequency oscillator circuit with a varying duty cycle is controlled by a variable resistor. The variable resistor is set according to the brightness level desired by the operator. The output of the oscillator is fed both to filament lamps and a brightness control circuit for the VFD. The brightness control circuit outputs a signal to the VFD that has a frequency and duty cycle corresponding to that which was also applied to the filament lamps.
U.S. Pat. No. 4,388,558 illustrates a circuit in which a fixed frequency oscillator is used to provide a variable duty ratio to various types of display devices such as filament lamps, VFDs and light emitting diode (LED) display devices. It is appreciated that the different types of devices respond differently to the selected duty ratio applied and therefore provide different levels of brightness when the variable resistor is adjusted. Analog circuits are described in the patent which modify the duty ratio applied to the VFD, as compared to that which is supplied to the filament lamps, and further modify the duty ratio that is applied to the LED, as compared to that which is applied to the VFD.
U.S. Pat. No. 4,704,560 describes a digital power supply for a VFD. In that system, a microprocessor contains a look-up table of stored codes that correspond to predetermined grid and anode voltages and is used in conjunction with a digital pulse width modulator circuit to supply the appropriate display voltage to an associated VFD device. The described circuit is said to provide automatic compensation of pulse width modulation to the VFD based upon loading effects of other energized segments within the display. The microprocessor uses the look-up table to supply coded values to the digital pulse width modulator circuit when loading changes occur due to changes in the number of preselected anode segments.